Slides - nanoHUB.org

NCLT July 13th, 2006
Nanotubes and Nanowires One-dimensional Materials
Tim Sands
Materials Engineering and Electrical &
Computer Engineering
Birck Nanotechnology Center
Purdue University
100 nm
Si/SiGe nanowires grown by the vapor-liquid-solid mechanism - Y. Wu et. al., Nano Lett 2:83 2002
Note: The diameter of these relatively large nanowires is 1/1,000th the diameter of a human hair!
Questions…
•
•
•
•
•
What is a nanowire? What is a nanotube?
Why are they (scientifically) interesting?
What are their potential applications?
How are they made?
???
Q: What is a nanowire?
• A: Any solid material in
the form of wire with
diameter smaller than
about 100 nm
Transmission
electron
micrograph of
an InP/InAs
nanowire
(M.T. Bjork et.
al., Nanoletters,
2:2 2002)
Q: What is a nanotube?
• A: A hollow nanowire, typically with a wall thickness on
the order of molecular dimensions
• The smallest (and most interesting) nanotube is the
single-walled carbon nanotube (SWNT) consisting of a
single graphene sheet rolled up into a tube
Scanning
Tunneling
Micrograph of a
single-walled
carbon nanotube
and corresponding
model (Dekker)
Q: How big is a nanometer?
••
••
••
••
11 billionth
billionth of
of aa meter
meter
1/50,000
1/50,000 the
the diameter
diameter of
of aa human
human hair
hair
40%
40% of
of the
the diameter
diameter of
of aa DNA
DNA molecule
molecule
55 times
times the
the interatomic
interatomic spacing
spacing in
in aa silicon
silicon
crystal
crystal
1 nm
PolySi image: C. Song, NCEM
10-2m
1 cm
10 mm
10-3m
1,000,000
nanometers=
1 millimeter
(mm)
The Nanoscale
Fly ash
~10-20 μm dia.
Human hair
10-50 μm dia.
Red blood cells
with white cell
2-5 μm dia.
10-4m
0.1 mm
100 μm
10-5m
0.01 mm
10 μm
The Nanoworld
10-6m
~10 nm dia.
Visible spectrum
Dust mite
~500 μm
The Microworld
Ant
~5 mm
1000
nanometers=
1 micrometer
(μm)
10-7m
0.1 μm
100 nm
10-8m
0.01 μm
10 nm
Head of a pin
1-2 mm
Microelectromechanical devices
10-100 μm wide
Red blood cells
Pollen grain
Assemble nanoscale
building blocks to make
functional devices,
e.g., a photosynthetic
reaction center with
integral semiconductor
storage
Nanotube devices (C. Dekker)
ATP synthesis
10-9m
DNA
2.5 nm dia.
10-10m
Atoms in silicon
0.2 nm spacing
1 nanometer (nm)
0.1 nm
Quantum corral of 48 iron atoms on
copper surface positioned one at a time
with an STM tip - Corral diameter 14 nm
Carbon nanotube
~2 nm diameter
Adapted from: NRC Report: Small Wonders, Endless Frontiers: Review of the National
Nanotechnology Initiative (National Research Council, July 2002)
Q: What makes nanowires and
nanotubes (scientifically) interesting?
• Electronic & optical properties
– Nanowires and nanotubes are the most confining electrical
conductors - puts the squeeze on electrons
– Can be defect free - electrons move “ballistically”
– Quantum confinement - tunable optical properties
• Mechanical properties
– Small enough to be defect-free, thus exhibiting ideal strength
• Thermal properties
– Can be designed to conduct heat substantially better (or
much worse) than nearly every bulk material
• Chemical properties
– Dominated by large surface-to-volume ratio
Nanowires and Nanotubes are New Materials!
Electronic and Optical Properties
Quantum Confinement
Electrons in solids
•
•
Core shell electrons are tightly bound to atoms, and
do not interact strongly with electrons in other atoms
Valence shell electrons are the outer electrons that
contribute to bonds between atoms
If all of the bonds are “satisfied”
by valence electrons, and if
these bonds are strong, then
the material does not conduct
electricity - an insulator
conduction
Electron energy
•
valence
core
Electrons in solids
If all the bonds are “satisfied”, but the bonds are relatively weak,
then the material is an intrinsic semiconductor, and thermal
energy can break a small number of bonds, releasing the
electrons to conduct electricity
Electron energy
•
conduction
valence
core
Electrons in solids
If impurities with one more or one fewer electrons than a host
atom are substituted for host atoms in a semiconductor, then the
material becomes conductive - an extrinsic semiconductor
Electron energy
•
conduction
valence
core
Electrons in solids
If only a fraction of the bonds are satisfied (or alternatively, if
there are many more electrons than are needed for bonding)
then there is a high density of electrons that contribute to
conduction, and the solid is a metal
conduction
Electron energy
•
valence
core
Electrons in nanostructured materials
• Electrons can be considered to be both waves
and particles
• Particle: momentum = mass x velocity (p =
mv)
• Wave: momentum = Planck’s
constant/wavelength (p = h/λ)
• Particle or wave: kinetic energy = p2/2m
Electrons in nanostructured materials
•
An electron (particle) can be described as a wave packet, i.e.
the sum of sinusoidal waves, each with a slightly different
wavelength
Electron Wave Packet
12
10
Sum of 10 cosine waves
with slightly different
wavelengths
8
6
4
2
0
-30
-20
-10
-2
0
-4
-6
-8
-10
Position
10
20
30
Electrons in nanostructured materials
•
•
•
•
Each wave (“wavefunction”) is analogous to the shape of a
vibrating string (or jump rope).
If we hold two ends of the string, we constrain the string to
vibrate within an “envelope” that contains an integer number of
half wavelengths…a “standing wave”.
Each half-wavelength exists between two “nodes” - points that
do not move
The shorter the wavelength (shorter string or more nodes), the
more energy it takes to vibrate the string
Envelope Function
Envelope Function
1.5
1.5
1
1
0.5
0.5
0
0
0
0.5
1
1.5
2
2.5
3
0
3.5
-0.5
-0.5
-1
-1
1
2
3
4
-1.5
-1.5
Position
Position
5
6
7
From vibrating strings to electrons
• An electron’s wavefunction is just like the vibrating
string
• If we move the nodes closer together, the electron’s
energy increases.
• If we put an electron in a “box”, the walls of the box
become nodes - so the smaller the box, the higher
the energy of the electron
Confining the electrons fundamentally changes
their electronic structure…
Quantum confining materials are NEW materials
with properties that are tunable with size
Optical properties
• Decreasing the size of a nanostructured
material increases the energy difference, ΔE,
between allowed electron energy levels
• When an electron drops from a higher energy
state to a lower energy state, a quantum of
light (“photon”) with wavelength, λ = hc/ΔE
may be emitted
• Larger ΔE implies shorter wavelength (“blue
shifted”)
h is Planck’s constant; c is the speed of light
Example: Quantum Dots
CdSe nanocrystals; Manna, Scher and
Alivisatos, J. Cluster Sci. 13 (2002)521
Leftabsorption
spectra
of
semiconductor
nanparticles
of
different diameter (Murray, MIT).
Right- nanoparticles suspended in
solution (Frankel, MIT) - National
Science Foundation Report, “Societal
Implications
of
Nanotechnology”
March 2001
Electrons in nanostructured materials
•
•
•
•
Each wave (“wavefunction”) is analogous to the shape of a
vibrating string (or jump rope).
If we hold two ends of the string, we constrain the string to
vibrate within an “envelope” that contains an integer number of
half wavelengths…a “standing wave”.
Each half-wavelength exists between two “nodes” - points that
do not move
The shorter the wavelength (shorter string or more nodes), the
more energy it takes to vibrate the string
Envelope Function
Envelope Function
1.5
1.5
1
1
0.5
0.5
0
0
0
0.5
1
1.5
2
2.5
3
0
3.5
-0.5
-0.5
-1
-1
1
2
3
4
-1.5
-1.5
Position
Position
5
6
7
Example: Quantum Dots
CdSe nanocrystals; Manna, Scher and
Alivisatos, J. Cluster Sci. 13 (2002)521
Leftabsorption
spectra
of
semiconductor
nanparticles
of
different diameter (Murray, MIT).
Right- nanoparticles suspended in
solution (Frankel, MIT) - National
Science Foundation Report, “Societal
Implications
of
Nanotechnology”
March 2001
Electrons in solids
If all the bonds are “satisfied”, but the bonds are relatively weak,
then the material is an intrinsic semiconductor, and thermal
energy can break a small number of bonds, releasing the
electrons to conduct electricity
Electron energy
•
conduction
valence
core
Questions and Answers
Electrons in solids
If all the bonds are “satisfied”, but the bonds are relatively weak,
then the material is an intrinsic semiconductor, and thermal
energy can break a small number of bonds, releasing the
electrons to conduct electricity
Electron energy
•
conduction
valence
core
Electrons in nanostructured materials
• Electrons can be considered to be both waves
and particles
• Particle: momentum = mass x velocity (p =
mv)
• Wave: momentum = Planck’s
constant/wavelength (p = h/λ)
• Particle or wave: kinetic energy = p2/2m
Another approach: nanoporous templates
side
side view
view
Pore
Pore bottoms
bottoms
top view
500 nm
Porous anodic alumina (PAA) with 50 nm diameter pores on 100 nm centers
produced by anodizing Al foil at 40V at 4°C in oxalic acid
How far can we go with silicon?
From “Extending Moore’s Law in the Nanotechnology Era” by Sunlin Chou (www.intel.com)
…or nano-tennis racquets…
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…or nano-tennis racquets…
Free Babolat Tee Shirt AND a $20 Instant
rebate on any Nanotechnology racket for a
limited time only!
New high Modulus Carbon Graphite Carbon Nanotube
Headsize:118 sq. in. * Length:28 in. * Weight (strung):9.3 oz.
* Stiffness (Babolat RDC):73 * Balance:14.63 in. Head Heavy *
Cross Section: 28mm Head/26mm Throat * Swingweight: 326 kg*sq.
cm * "This year, Babolat introduced two Nano Carbon Technology,
or NCT, racquets, the Tour and the Power. The Tour, above is
geared toward high-level intermediates while the Power (below) is
for those with short strokes. Both models are reinforced in the throat
and the bottom half of the head with carbon nanotubes, strings of
carbon molecules constructed at the nano level, which are five times
stiffer than graphite. Net result: Babolat is able to increase the
stiffness and power of the racquets without adding weight."––
Racquet Sports Industry.